The present invention relates to radioactivity monitoring apparatus and seeks to improve radioactivity measurement accuracy and sensitivity of detection in the presence of interfering isotopes.
When, for example, dust is collected on a filter medium and monitored for alpha and/or beta particles from long-lived uranic/transuranic elements, radioactivity measurements are significantly affected by unwanted isotopes, such as naturally occurring radon and thoron progeny. This interference is normally reduced by applying alpha spectrometry but energy degradation in the air gap between filter and detector results in distortion of the spectrum making it difficult to completely separate the isotopes. Attempts have been made to counter these effects by examining the shape of the energy spectrum and estimating the degree of interference but significant errors can still arise when, for example, rapid changes in environmental conditions produce rapid variations in the degree of spectral distortion caused by the air gap.
We have realised that these errors can be significantly reduced by predicting the change in spectral distortion and altering the compensation in order to more completely remove the effect of the interfering isotopes. A priori knowledge of the conditions obtaining in the air gap at the time of measurement can be derived from sensors incorporated into the instrument or information fed to it from external sources.